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A girl-on-girl tryst featuring Playboy Playmate Karissa Shannon and Heidi Montag is among the sex tape library The Hills star's estranged husband is planning to go public with.

Spencer Pratt revealed he is in negotiations to sell the footage of his wife having sex to Internet porn giant Vivid Entertainment last week, and now TMZ.com reports one extract features Hugh Hefner's girlfriend Shannon.

Sources tell the website Pratt came upon the footage while searching through boxes of belongings at his Malibu, California home.

The busty model tells TMZ.com that such a tape does exist - buts she doubts Pratt has it.

And, if he does go through with his plans to sell the sex film as part of a deal with Vivid Entertainment boss Steven Hirsch, she has threatened to sue.

At press time, Shannon had not made an official comment on the sex tape.